The present invention relates to a new method for preparing a crystalline aluminum orthophosphate (AlPO.sub.4), particularly a fine particulate crystalline aluminum orthophosphate.
Crystalline AlPO.sub.4 has been used as the raw material of optical glasses and various ceramics, as a catalyst, etc. Further, since AlPO.sub.4 has a crystal structure similar to that of rock crystal (SiO.sub.2), it has recently been considered promising as a vibrator material (piezoelectric material) like LiTaO.sub.3 and rock crystal.
Conventional crystalline AlPO.sub.4 has been produced by the following methods:
(1) Mixing aluminum hydroxide or aluminum oxide with orthophosphoric acid and calcining the mixture. (Calcining Method)
(2) Reacting an aqueous solution of a soluble phosphate with an aqueous solution of an aluminum salt and obtaining aluminum phosphate as a precipitate. (Precipitating Method)
However, according to the above calcining method (1), it is necessary that a solid product obtained by reacting aluminum hydroxide or aluminum oxide with phosphoric acid at a temperature of about 130.degree.-270.degree. C. be further calcined at a high temperature of 300.degree.-1,300.degree. C. for 5 to 20 hours. Consequently, this calcining method involves problems such that the manufacturing time becomes longer and the consumption of heat energy is increased and that the reaction apparatus requires an expensive material having corrosion resistance to phosphoric acid which is heated to a high temperature. According to the above precipitating method (2), an aqueous solution of a phosphate of, for example, sodium, potassium or ammonium and an aqueous solution of an aluminum salt such as, for example, aluminum sulfate are reacted together to obtain aluminum phosphate as a precipitate. However, the aluminum phosphate obtained by this precipitating method is found to be amorphous by X-ray diffraction, and in order to obtain a crystalline AlPO.sub.4, it is necessary to calcine this precipitate at a temperature above 1,000.degree. C. Thus, according to either of the above conventional manufacturing methods, it is necessary to perform calcination at a high temperature in order to obtain a crystalline AlPO.sub.4. Additionally, in such known manufacturing methods, it is unavoidable that impurities incorporate into product due to corrosion of the material of the apparatus in the calcining step. Thus, those methods cannot fully satisfy the requirements of the electronic industry and other recent industrial techniques which require a specially high purity.